1. Field of the Invention
The present invention relates to a lead frame which is made of Ni, an Ni alloy, Cu, a Cu alloy, Fe or an Fe alloy and used for ICs, etc., especially, to the semiconductor device with the lead frame for electronic parts, which does not comprise Pb being one of harmful contaminants and pollutants for environment, and a manufacturing method therefor.
2. Description of the Prior Art
FIGS. 3 and 4 are a plane view and a sectional view of a conventional lead frame, respectively. In case of the conventional lead frame, inner lead parts 2, to which wires are bonded, have been plated with Ag, and outer lead parts 1 have been subjected to a surface treatment with a coating material containing Pb and Sn.
In recent years, the environmental problems focus the spotlight of attention so that, regarding also components of IC packages, materials which do not contain any detrimental contaminants and pollutants for environment have been examined and looked for.
Among substances which are contained in lead frames of electronic parts, lead is an especially harmful pollutant to the environment. Lead in the lead frames, which are left in the environment, will dissolve into the environment to adversely affect on health of human bodies. Thus, the electronic industry tries to develop a Pb-free solder, a Pb-free paste solder and so on. But, there is no new Pb-free solder at present, which has the same or better characteristics than conventional Pb-containing solders, in the stage of actual application.
Regarding the lead frame of electronic parts, various efforts have been made for developing a new one. Recently, in order to avoid to use Pb-containing solders for lead frames, there has been proposed and practically used a lead frame whose surface is overall plated with palladium (Pd). But, such a palladium-nickel lead frame has a problem that, when the lead frame is heated up in the die-bonding process (or die-attach process) or the wire-bonding process, the Pb-free solder is deteriorated in wettability to the plated palladium resulting in low reliability of solder joints. Therefore, recently, it has been proposed a palladium-nickel lead frame plated with gold (Au) as a protective film. However, because only a limited number of countries can supply palladium, the price thereof is liable to rise due to its short supply so that there arises a cost problem. In the case of providing the protective film of gold on the palladium plating, the cost problem become further big.
Further, regarding the palladium-nickel frame, since burrs are liable to be generated in the resin molding process when assembling ICs, an additional process of removing the burrs is required resulting in an increase of manufacturing cost.
On the other hand, in the case of a lead frame whose overall surface is plated with palladium, since a large potential difference is generated between palladium and the base metal of the lead frame, even if an intermediate layer of Ni or a Pdxe2x80x94Ni alloy is provided between the lead frame made of Ni, a Ni alloy, Fe or an Fe alloy, for example, and the palladium plating, there arises a lack of reliability with regard to a corrosion problem. Thus, the palladium plating can be applied to only a lead frame made of Cu or a Cu alloy.
There has been conducted another development of lead frames than those with utilization of palladium, which provides a Pb-free solder plating which includes indium (In), bismuth (Bi), zinc (Zn), etc. as an alternative to lead in the usual Pbxe2x80x94Sn system solder. With regard to a solder alloy or a paste solder for the reflow process, there have been proposed alloys each of which includes two or more alloying elements as well as tin (Sn), namely each of which consists of three or four alloying elements including Sn. But, since it is difficult to control the plating alloy composition consisting of three or four alloying elements, which is deposited from the plating solution, binary alloys consisting of Sn and another alloying element have been mainly tried to develop.
However, an Inxe2x80x94Sn alloy is inappropriate for practical use because indium is expensive.
With regard to a Bixe2x80x94Sn alloy, while it is possible to make its melting point low, it is inappropriate for lead frames, which are subjected to a bending process during manufacturing, because of bad formability due to becoming hard and brittle when lowering the melting point thereof. Further, the Bixe2x80x94Sn alloy has a defect that the lift off phenomenon occurs, in which ICs float on the solder in the surface-mounting process, because it has low bonding strength and low thermal fatigue strength due to low wettability.
A Znxe2x80x94Sn alloy has the melting point close to that of the conventional Pbxe2x80x94Sn solder alloy and an advantage that zinc is cheap. However, the Znxe2x80x94Sn alloy has a defect that, because zinc is liable to be oxidized in air, it oxidizes when receiving thermal hysteresis in the assembling process so that it is deteriorated in wettability.
Recently, on the other hand, there has been proposed an Agxe2x80x94Sn alloy as the most hopeful Pb-free solder and developed plating solutions therefor. However, there are barriers for practical use thereof since it have some problems of appearance, occurrence of cracks when bending, a tarnish due to thermal hysteresis, deterioration of wettability due to thermal hysteresis and so on.
An object of the invention is to solve the above mentioned problems of the known solders and provide a semiconductor device and a manufacturing method thereof wherein the semiconductor device comprises a surface treatment layer as a post-plating which includes Ag and Sn and which is provided on a low cost lead frame for electronic parts mounted on the semiconductor device, and wherein the plating does not comprise lead (Pb) which is one of harmful contaminants and pollutants for environment, and has good soldering wettability and good bonding strength property.
Under the object, there is provided a lead frame made from Ni, a Ni alloy, Cu, a Cu alloy, Fe or an Fe alloy, which comprises an inner lead part with a surface treatment layer of Ag or a Ag-containing alloy and an outer lead part with a surface treatment layer of an alloy containing Ag and Sn, wherein the surface treatment layer has a brightness of not less than 0.6 and Sn has the body-centered tetragonal lattice with the crystal orientation indices of from 1.5 to 5 at the (220) plane, not more than 0.9 at the (211) plane and not less than 0.5 at the (200) plane.
In order to form the surface treatment layer containing Ag and Sn, which has a brightness of not less than 0.6 and in which Sn has the body-centered tetragonal lattice with the crystal orientation indices of from 1.5 to 5 at the (220) plane, not more than 0.9 at the (211) plane and not less than 0.5 at the (200) plane, a specific plating solution is used, which contains, at least, one or more selected from alkane sulfonic acid, alkanol sulfonic acid and sulfamine acid as the acid component, one or more of tin methane-sulfonate and SnO as a tin salt, and one or more selected from silver methane-sulfonate, Ag2O and AgO as a silver salt.
In order to improve the bonding property between the base of the outer lead part and the surface treatment layer, the outer lead part is preliminarily treated prior to forming the surface treatment layer by a treating agent of one or more selected from hydrochloric acid, nitric acid and sulfonic acid.
Further, in order to improve surface conditions and soldering wettability of the outer lead part, the formed surface treatment layer is treated by a treating agent containing sodium triphosphate.
The surface treatment layer may have a thickness of preferably 3 to 15 xcexcm.
The alloy composing the surface treatment layer may comprise 1 to 8 wt % Ag, thereby the defects of the known art can be totally improved so that there can be provided a low cost lead frame for electronic parts and a manufacturing method thereof, the lead frame comprising no lead (Pb), which is one of harmful contaminants and pollutants for environment, and having good soldering wettability and good bonding strength property.